Individuals with a ICD10 F32-F33 diagnosis in 2016 or earlier were considered as depression cases and the random population-based sample excluding individuals with a depression diagnosis were used as controls. All individuals diagnosed with bipolar disorder were excluded, except for the analyses involving co-occurrence of depression and bipolar disorder diagnosis. Depression cases were divided into two groups; 1) individuals diagnosed with a single episode of depression, and 2) those fulfilling the criteria for recurrent depression. The date of the first depression episode for each individual was defined as the start date of the first contact in the Danish Psychiatric Central Research Register with an ICD-10 code of F32-F33 diagnoses at an age of 10 years or older. When defining later depression episodes only contacts with the ICD-10 codes: F32, F33.0-F33.3, F33.8-F33.9 were considered. Individuals with the ICD-10 code F33.4 “Major depressive disorder, recurrent, in remission” were omitted because these contacts are unlikely to indicate a new episode of depression. Only ICD-10: F32, F33.0-F33.3, F33.8-F33.9 diagnosis at dates later than 60 days after the end date for all previous ICD10: F32-F33 diagnosis were considered as new episodes of depression, thus categorizing that individual as having recurrent depression.
